Background Trichomonas vaginalis (T. vaginalis ) belongs to a common sexually transmitted disease pathogen causing genitourinary trichomoniasis in both sexes. We investigated the pathogenetic mechanism of gen itourinary trichomoniasis.Methods Cultured T. vaginalis bodies were injected into the vaginas of rats, or incubated with genitourinary epithelial cells of female subjects, male subjects, and sperm. The ultrastructural and microscopic changes were observed via transmission and scanning electron microscopy and through microscopic histochemistry.Results Groups of T.vaginalis adhered to PAS positive colum nar cells at the surface of stratified epithelium in the middle and upper portions of the vaginas. They a lso traversed under these cells. The parasites were shown to be PAS, cathepsin D, and actin positive, and they could release hydrolase into the cytoplasm of adher ed epithelial cells. In the amebiform T.vaginalis, microfilaments were arranged into reticular formation. Similar phenomena were found during the interaction of T.vaginalis with host cells, both in vitro and in vivo. Usually several protozoa adhered to a n epithelial cell and formed polymorphic pseudopodia or surface invaginations to surround and phagocytize the microvilli or other parts of the epithelial cytoplasm. Adhesion and phagocytosis of sperm by the protozoa occurred at 15-30 minutes of incubation. Digestion of sperm was found at 45-75 minutes and was complete at 90-105 minutes.Conclusions T.vaginalis tends to parasitize at the fornix o f the vagina, because this is the site where columnar cells are rich in mucinogen granules and thei r microvilli are helpful for adhesion and nibbling. T.vaginalis possesses some invad ing and attacking abilities. Shape change, canalization, encystation, phagocytosis, digestion, the cell coat, cytoskeleton, and lysosome all play important roles in the process of adhesion. They have two methods of phagocytosis: nibbling and ingestion. Genitourinary epithelium may be injured directly by the digestive action of hydrolases, phagocytosis, and the mechanical action of pseudopodia. Background Trichomonas vaginalis (T. vaginalis) belongs to a common sexually transmitted disease pathogen causing genitourinary trichomoniasis in both sexes. We investigated the pathogenetic mechanism of gen it it canine trichomoniasis. Methods Cultured T. vaginalis bodies were injected into the vaginas of rats, or incubated with gen ultrasurgial epithelial cells of female subjects, male subjects, and sperm. The ultrastructural and microscopic changes were observed via transmission and scanning electron microscopy and through microscopic histochemistry. Results Groups of T. vaginalis adhered to PAS positive colum nar cells at the surface of stratified epithelium in the middle and upper portions of the vaginas. They a lso traversed under these cells. The parasites were shown to PAS, cathepsin D, and actin positive, and they could release hydrolase into the cytoplasm of adher ed epithelial cells. In the amebiform T.vaginalis, microfilaments were arranged into reticular formation. Similar ph enomena were found during the interaction of T. vasinalis with host cells, both in vitro and in vivo. Both several protozoa adhered to an epithelial cell and formed polymorphic pseudopodia or surface invaginations to surround and phagocytize the microvilli or other parts of the epithelial cytoplasm. Adhesion and phagocytosis of sperm by the protozoa occurred at 15-30 minutes of incubation. Digestion of sperm was found at 45-75 minutes and was complete at 90-105 minutes. Conlusions T. vasinalis tends to parasitize at the fornix of the vagina, because this is the site where columnar cells are rich in mucinogen granules and thei r microvilli are helpful for adhesion and nibbling. T. vaginalis possesses some invad ing and attacking abilities. Shape change, canalization, encystation, phagocytosis, digestion, the cell coat, cytoskeleton, and lysosome all play important roles in the process of adhesion. They have two methods of phagocytosis: nibbling and ingestion. Genitourinary epitheliummay be injured directly by the digestive action of hydrolases, phagocytosis, and the mechanical action of pseudopodia.